Lenny's point is true - except for the danger of setting that mca param and its possible impact on ORTE daemons+mpirun - see other note in that regard. However, it would only be useful if the same user was doing it.

I believe Tim was concerned about the case where two users are sharing nodes. There is no good solution for that case. Two mpiruns done by different users that share a node and who have no knowledge of the other's actions will cause collision.

We should probably warn about that in our FAQ or something since that is a fairly common use-case - only thing I can think of is to recommend people default to running without affinity and only set it when they -know- they have sole use of their nodes.

On Jul 29, 2008, at 12:17 AM, Lenny Verkhovsky wrote:

for two separate runs we can use slot_list parameter ( opal_paffinity_base_slot_list ) to have paffinity

Actually, this is true today regardless of this change. If two separate mpirun invocations share a node and attempt to use paffinity, they will conflict with each other. The problem isn't caused by the hostfile sub-allocation. The problem is that the two mpiruns have no knowledge of each other's actions, and hence assign node ranks to each process independently.

Thus, we would have two procs that think they are node rank=0 and should therefore bind to the 0 processor, and so on up the line.

Obviously, if you run within one mpirun and have two app_contexts, the hostfile sub-allocation is fine - mpirun will track node rank across the app_contexts. It is only the use of multiple mpiruns that share nodes that causes the problem.

Several of us have discussed this problem and have a proposed solution for 1.4. Once we get past 1.3 (someday!), we'll bring it to the group.

On Jul 28, 2008, at 10:44 AM, Tim Mattox wrote:

My only concern is how will this interact with PLPA. Say two Open MPI jobs each use "half" the cores (slots) on a particular node... how would they be able to bind themselves to a disjoint set of cores? I'm not asking you to solve this Ralph, I'm just pointing it out so we can maybe warn users that if both jobs sharing a node try to use processor affinity, we don't make that magically work well, and that we would expect it to do quite poorly.

I could see disabling paffinity and/or warning if it was enabled for one of these "fractional" nodes.

Per an earlier telecon, I have modified the hostfile behavior slightly to allow hostfiles to subdivide allocations.

Briefly: given an allocation, we allow users to specify --hostfile on a per-app_context basis. In this mode, the hostfile info is used to filter the nodes that will be used for that app_context. However, the prior implementation only filtered the nodes themselves - i.e., it was a binary filter that allowed you to include or exclude an entire node.

The change now allows you to include a specified #slots for a given node as opposed to -all- slots from that node. You are limited to the #slots included in the original allocation. I just realized that I hadn't output a warning if you attempt to violate this condition - will do so shortly. Rather than just abort if this happens, I set the allocation to that of the original - please let me know if you would prefer it to abort.

If you have interest in this behavior, please check it out and let me know if this meets needs.